The invention relates generally to solid state lasers and more particularly to Q-switched lasers for producing high peak power pulses.
Q-switching is a technique used to obtain high peak power laser pulses. Q-switching is performed by modulating the losses in a laser cavity. When cavity loss is high, pumping energy is stored in the laser gain medium by building up the population inversion; the high cavity loss prevents laser action from occurring which would deplete the stored energy. The stored energy is then extracted in a high peak power pulse by reducing the cavity losses. The use of an acousto-optic modulator inside a laser cavity for Q-switching is described in Chang, "Acousto-optic Devices and Applications," IEEE Transactions on Sonics and Ultrasonics, Vol. SU-23, No. 1, January 1976 on page 17. However, as noted therein nearly all acousto-optic Q-switches are made of fused silica. Another concern mentioned therein is the substantial RF power that must be applied to the Q-switch to prevent lasing.
U.S. patent applications Ser. No. 730,002 filed May 1, 1985, now U.S. Pat. No. 4,653,056 issued Mar. 24, 1987 and Ser. No. 811,546 filed Dec. 19, 1985 now U.S. Pat. No. 4,656,635 issued Apr. 7, 1987 describe a class of solid state lasers which are laser diode pumped in a longitudinal pumping configuration and which can be pumped in the TEMOO mode. These solid state lasers can be made extremely compact. U.S. patent application Ser. No. 864,928 filed May 19, 1986, now U.S. Pat. No. 4,665,529 issued May 12, 1987 describes a fiber optic coupled longitudinal pumping scheme which provides further miniaturization of these solid state lasers. In these lasers mode matching of the laser diode pumping source to the active mode volume of the laser cavity provides for high gain in a small volume.
These solid state lasers are efficient wavelength and mode converters for diode lasers. Diode pumped lasers do not use cooling water and do not rely on broadband excitation sources. Therefore they are not subject to water or flashlamp induced noise and exhibit greatly reduced thermal lensing. These characteristics result in excellent beam pointing stability and pulse-to-pulse stability. It is desirable to obtain these advantages in a Q-switched laser.
The typical prior art Q-switched solid state laser cannot produce the desired combination of peak power and short pulse width. A long laser rod is pumped by tungsten or arc lamps. A fused silica Q-switch is also placed in the laser cavity and powered from an RF source connected to a LiNbO.sub.3 transducer mounted to the silica substrate. The cavity is relatively long, typically about 1 foot and the Q-switch is large.
To form short pulses, either a short cavity or increased gain in the cavity is required, since pulse width depends on the product of gain and cavity round trip time. For a 1 foot cavity, the round trip time is about two ns. To obtain high gain in the cavity to compensate for the long cavity length, the gain medium must be very highly pumped. Typically several KW of power are supplied to the arc lamp used to pump the laser rod (since pumping efficiency is only about 5%). However the higher gain required to obtain a desired pulse width can result in too high a pulse intensity since the total energy (product of power and pulse width) in the pulse is approximately constant and peak power increases as pulse width decreases. Therefore there is a tradeoff in obtaining the pulsewidth-energy desired. Thus, the usual results are the right peak power but too long a pulse or a sufficiently short pulse at too high peak power.
It is desirable to produce a more effective short pulse Q-switched solid state laser which provides a short pulse at low energy by using a short cavity at moderate gain instead of a longer cavity at very high gains. The aforementioned class of solid state lasers, described in the above cited patent applications, allow the resonator to be made small, typically about 1 inch long, while the efficient longitudinal diode-pumped arrangement (about 30% efficient) provides for a reasonable gain. The output can also easily be produced in TEMOO mode which is useful for many applications. Thus it will be desirable to incorporate a Q-switch into this type of solid state laser so that very short pulses can be produced at desired low energy levels but still with relatively high peak power. It is also desirable to utilize nonstandard materials for the Q-switch in order to produce a miniaturized Q-switch which fits into such a short cavity with low power pumping requirements.
Light transmitted through an optical fiber is red shifted by stimulated Raman scattering; however, the process presently requires very high poewr and expensive lasers. A practical source of sufficiently high power short pulses is needed to produce Raman conversion. It is thus desirable to utilize short high peak power pulses produced by a compact Q-switched laser diode pumped solid state laser for continuum generation in an optical fiber.